DESCRIPTION
This invention relates to infrared detectors comprising a chamber holding a vacuum space and to methods of producing a vacuum device (particularly but not exclusively an infrared detector), in which a chemically-active getter is provided to getter gases from the evacuated chamber.
Published United Kingdom patent application GB-A-2 179 785 describes a known method of producing and maintaining a vacuum space in a chamber of a vacuum device (for example an electron discharge device such as a ring laser gyroscope), comprising the steps of (a) providing a porous getter in an evacuation tube which forms an outlet of the chamber, (b) evacuating the chamber to produce the vacuum space by means of a vacuum pump connected to the evacuation tube, (c) heating the getter in the evacuation tube so as to activate chemically the getter material and (d) disconnecting the vacuum pump and sealing the evacuation-tube outlet of the chamber while leaving the chemically-active getter in communication with the vacuum space of the chamber to getter gases from the evacuated chamber. By locating the getter within the tubulation used to evacuate the device, problems which otherwise arise in trying to accommodate the getter within the device chamber are avoided. The whole contents of GB-A-2 179 785 are hereby incorporated herein as reference material.
According to GB-A-2 179 785 it is necessary to provide the getter device with a support element to position it within the tubulation. In order to avoid the need for a separate support element, GB-A-2 179 785 describes the provision of the getter as an electrophoretically-deposited layer of porous sintered non-evaporable getter material selectively deposited on the internal surface of the evacuation tube. Getter-free zones are present at each end on the internal surface of the tube. Such a getter also avoids problems of a separately-supported getter becoming detached from its required position or producing loose fragments in the presence of shocks or vibrations.
Infrared detectors are not mentioned in GB-A-2 179 785. They often comprise a vacuum space between inner and outer walls of a dewar envelope. At least one infrared detector element is mounted on an end of the inner wall, normally within the vacuum chamber between the inner and outer walls. The detector elements which are delicate and small need to be protected against damage and must not be obscured to block incidence of the infrared radiation to be detected. The inner wall defines an inner chamber accommodating a cryogenic cooling element which serves to cool the inner wall end and hence the detector element thereon, during operation of the detector. The cooled inner wall is often termed the "cold finger" of the detector.
A major cause of infrared detector failure is the gradual degeneration of the vacuum in the space between the inner and outer walls due to internal out-gassing of the various component parts of the detector exposed to the vacuum. In order to reduce the effects of internal out-gassing it is known to provide at least one getter in the vacuum space for gettering gas molecules from this space. U.S. Pat. No. 3,851,173 describes one example of an infrared detector incorporating a getter to maintain a vacuum in a dewar. A non-evaporable chemically-active SAES getter is mounted on the outer wall and in the vacuum space between the outer wall and the cold finger. Such chemical getters are activated by being taken to a high temperature after evacuating and sealing the dewar envelope. This is normally achieved with an electrical heating element embedded in the getter material formed as a unit with electrical connection leads (not specifically shown in U.S. Pat. No. 3,851,173) passing through vacuum-tight seals in the dewar. U.S. Pat. No. 4,206,354 shows an example of such a dewar getter with its connection leads.
Some protective measure is required between this type of getter and the inner wall, the detector element and its connections in/on the inner wall, all of which could otherwise be damaged by the high activation temperature. This may be achieved as described in U.S. Pat. No. 3,851,173 by using an extra heat-shield member which surrounds a substantial portion of the inner wall to protect the cold finger from the action of the getter. These factors can increase the cost of an infrared detector and may lead to an increased size for the dewar envelope, perhaps even the adoption of an unconventional dewar envelope outline. The heating also produces by-products including some outgassing of the heated parts of the dewar and so adds gases to the vacuum space.